Upload dropping down as low as 1.2mbps

Hello, I've been having issues with streaming to Twitch for several months. Nearly every day, my upload will drop from my paid-for 35mbps (I'm on the 1.2gbps download/35mbps upload plan) down to the low teens (11-14mbps upload) and usually will continue to drop down to 0, before spiking back up 20~ish mbps. It will usually repeat this a few times, then stabilize for a bit.

However, on some days, it will continuously bounce back and forth between 0-14mbps and never recover, making it impossible to deliver a decent experience to my stream watchers. 

My download is fine. 

I've had several techs come out, we've replaced all of our interior cables, replaced the modem two times (I'm currently on the latest modem), a tech replaced the outside wire (from the pole to the house) and some connections outside. All devices in the home experience this issue with the upload speed, from the PCs to the phones. The PC used for streaming is connected via ethernet cord (a new cat6) into the 2.5g connection of the new modem. 

I can't think of anything else I can do on my end. Can anyone help?